hi
You don't say what size the roof size is. Is It a shed, garage single/double or extension. I'm no roofer but I've worked In construction for over 30 yrs groundworks/bricklayer but I did have a mate who was a flat roofer and I've given him a hand a couple of times and I know he charged a couple of thou for a single garage roof felt with lime chippings. My advice would be to see If they have done any work In your area and check It out.
Just remember the old sayings
If It's to good to be true, It probally Is
&
You get what you pay for
HTH